Add test for array_diff()

[andrerademacher]

There are already unit tests for several PHP array functions, but some are still missing.
One of these is array_diff(). Variants like array_diff_xyz() are NOT part of this issue and will be implemented separately.

Examples:
array_merge() >> Unittest array_merge()
array_fill() >> Unittest array_fill()

ToDo:

• Read the example unit tests for array_merge() and array_fill(). Some edge cases that you will encounter are already solved here.
• Create a new unit test file zce71/tests/Arrays/ArrayDiffTest.php using the array_merge() example as template.
• Collect interesting edge cases for array_diff's behaviour and implement these inside the data provider(s).

Hint: The debugger is your friend. Sometimes, one learns to know some surprising "new" behaviour of PHP functions.